RU-ALLRUSSIAN 2018 Problem 114

On the sides $AB$ and $AC$ of the triangle $ABC$ , the points $P$ and $Q$ are chosen, respectively, so that $PQ\parallel BC$ . Segments $BQ$ and $CP$ intersect at point $O$ . Point $A'$ is symmetric to point $A$ relative to line $BC$ . The segment $A'O$ intersects the circumcircle $w$ of the triangle $APQ$ at the point $S$ . Prove that circumcircle of $BSC$ is tangent to the circle $w$ .